Production of a metallic product by electrolysis may be illustrated by reference to the production of aluminum. The commercial production of aluminum is typically accomplished by the Hall-Heroult process. In this process, the purified source of alumina is dissolved in a molten all-fluoride salt solvent particularly consisting of cryolite, and then reduced electrolytically with a formed carbon anode according to the reaction: EQU 1/2Al.sub.2 O.sub.3 +3/4C+3e.sup.- .fwdarw.Al+3/4CO.sub.2 EQU 1/2Al.sub.2 O.sub.3 +3/2C+3e.sup.- .fwdarw.Al+3/2CO.
Four characteristics of the Hall-Heroult process include: (1) carbon dioxide being produced and a carbon anode being consumed at the rate of 0.33:1 pound of carbon per pound of aluminum produced, which results in a required continual movement of the formed carbon anode downwardly toward the cathode aluminum pool at the bottom of the cell to maintain constant spacing for uniform aluminum production and thermal balance in the cell; (2) the need to feed intermittently and evenly solid alumina in a limited concentration range to the open cell to maintain peak efficiency of operation and in order to avoid the anode defects; (3) severe corrosion of the cell materials due to high temperatures of 950.degree. to 1000.degree. C. in the fluoride bath resulting in low cell life and increased labor; and (4) cell power efficiency is limited to less than 50% since a carbon-anode-to-liquid-aluminum distance greater than one inch must be maintained to reduce the magnetic field's undulation of the aluminum layer, which would cause intermittent shorting and result in Faradic losses due to a back reaction of aluminum droplets with carbon dioxide to produce alumina.
It is known that, in the Hall-Heroult cell reaction, the carbon of the anode contributes to the overall reaction of winning aluminum by decreasing the decomposition voltage. For example, the decomposition of Al.sub.2 O.sub.3 in cryolite on a platinum anode requires about 2.2 volts, but on a carbon electrode the decomposition voltage is about 1.2 volts.
In the all-fluoride-containing bath, the alumina will dissolve in the cryolite-fluoride salt bath at a temperature of 950.degree. to 1000.degree. C. Bayer alumina is soluble in a cryolite-containing bath at a temperature of at least 900.degree. C. A fluoride-containing bath having a temperature below about 900.degree. C. will not readily solubilize ordinarily processed Bayer alumina and, therefore, the alumina, as a source of aluminum, cannot enter the reduction reaction nor is it possible for the aluminum to be deposited at the cathode.